Optical devices that guide light along an optical path are found in many consumer and industrial applications. One application that is currently an area of significant interest is Augmented reality (AR) device technology. Examples of AR device technology include augmented reality headsets. For such platforms, it may be desirable to have a display device that provides red-green-blue (RGB) colors (e.g., full color) in a very compact package.
Optical elements redirect light as it is conveyed along a platform's optical path. The power efficiency of a display device, for example, may be improved through the design of the optical elements that convey the light from a source to a display screen or viewport. Optical elements may expand or reduce light emitted from the source to best match etendue of another component, for example. Refractive optical elements are relatively large. A typical micro convex lens may have a thickness of around 5 mm, for example. The use of conventional refractive optical elements may therefore constrain scaling of optical devices.
Flat optical elements can be much thinner than conventional optical elements. For example, flat lenses having thicknesses of only tens or hundreds of nanometers (nm) have recently been developed. A flat lens is therefore substantially two-dimensional, and can yield diffraction-limited optical performance. FIG. 1A illustrates a system 100 employing a flat lens 110. As shown, flat lens 110 includes a metasurface 120. Metasurface 120 includes a plurality of surface structures spatially arrayed over a substrate 315. Substrate 315 is typically polished quartz, or another material having similar flatness. These nanostructures may also be referred to as nanoantennas because they are each capable of functioning as a resonant optical antenna, which allows metasurface 120 to manipulate optical wave-fronts. For example, metasurface 120 may induce a phase delay, which may be precisely tuned over a footprint of the array. The controlled phase delay redirects light 105 transmitted through flat lens 110 to a focal point 180. Metasurfaces therefore provide opportunities to realize virtually flat, aberration-free optics in form factors much smaller than those required by geometrical optics.
While flat optical elements can be tuned for specific wavelengths of light by changing the size, shape or spatial layout of the nanostructures, a given design will work perfectly (i.e., be diffraction limited) for only the target wavelength. Hence, flat lens 100 will have a different focal length for different wavelengths. As further illustrated in FIG. 1B, metasurface 120 redirects light 106 of multiple wavelengths to multiple focal points 180, 185 and 190. The three different focal points are a result of light 106 having three different wavelengths with the difference in focal distance between focal points 180, 185 and 190 being a function of the bandwidth of light 106. Depending on the design of the metalens, and the material characteristics, longer wavelengths may have a shorter focal length, or they may be engineered to have a longer focal length. Regardless, incident light of a wider band will converge over a wider range of focal distances. As such, optical power outside of a given wavelength may be lost as a result of chromatic aberration in optical elements employing metasurfaces. It would be advantageous if optical elements employing metasurfaces could be made more suitable for optical platforms employing broadband light. Such broadband, flat optical elements may, for example, enable a significant reduction in the form factor of AR devices that operate over the visible light band. It would also be advantageous to reduce the cost of fabricating optical elements employing metasurfaces, whether narrowband or broadband.